In 796, the caliph Harun al-Rashid moved the seat of the Muslim Empire to the northern Syrian city of Raqqa, on the banks of the Euphrates. There, Harun — the inspiration for a caliph in “One Thousand and One Nights” — is said to have played polo and perfumed the air with garments worn by female slaves. Raqqa’s later history largely registers as more mundane, a tale of declines and revivals that turned catastrophic when the Islamic State, or ISIS, seized it in 2014, making the city its effective capital. American-backed forces are currently battling ISIS for control of a city in ruins.

“City of Ghosts,” a blunt instrument of a documentary, is an urgent and heart-heavy account of Syrian resistance from the ground up. Specifically, it recounts the story and activism of Raqqa Is Being Slaughtered Silently, or R.B.S.S., a citizen-journalist initiative that emerged in April 2014 to secretly record the atrocities committed by ISIS in Raqqa. (R.B.S.S.’s website states that the group is nonpartisan and explains that it is “a campaign launched by a group of nonviolent activists in Raqqa to expose the atrocities committed by the regime of Bashar al-Assad and terrorist extremist group the Islamic State of Iraq and Syria” toward civilians in that city.)

Directed by Matthew Heineman, “City of Ghosts” primarily focuses on three R.B.S.S. activists identified by their first names — Aziz, Hamoud and Mohamad — who, even after being forced to flee Raqqa, intently work to get images and information about the city out into the world. Mr. Heineman, who is also the movie’s cinematographer, more or less opens and closes the documentary in 2015 with some of the activists in New York, where R.B.S.S. received an award from the nonprofit group the Committee to Protect Journalists. It’s a distracting framing device that suggests Mr. Heineman believed that he first needed to give his audience reassuringly recognizable faces. Cut to David Remnick, editor of The New Yorker, at the podium.

“City of Ghosts” is best when it lets the activists tell their own stories, which they do in smoothly edited tag-team voice-over, in on-camera scenes from their life abroad and in ghastly underground images recorded in Raqqa. It’s an account of heroic resistance that essentially begins in 2011 when the Arab Spring reached Syria (“we started to scream for freedom”), and peaceful demonstrations against President Assad were met with state violence followed by power grabs. Along the way, Mr. Heineman furnishes a sketchy look at the various players and geopolitical stakes, glossing over Syria’s historical complexities and the role that the United States has played in the rise of ISIS.

The activists come across as ordinary men made extraordinary by circumstance; some seem almost surprised at where life and war have taken them. Aziz was a student who became a spokesman, and Mohamad was a teacher turned reporter, but not much else about the group’s origins comes through. Mr. Heineman glides over the nuts and bolts of its operational beginning. He may not have thought it germane, or maybe thought covering it might have endangered lives. Or perhaps Mr. Heineman, an excitingly visceral director — his last documentary was “Cartel Land” — thought it might slow this fast-paced feature down. Like the Arab Spring represented in the movie, R.B.S.S. almost seems to have spontaneously erupted.

That’s unfortunate because more information and in-depth analysis, as well as greater restraint in the use of atrocity images, might have deepened a movie that leans on shortcuts and visual shocks. In “City of Ghosts,” Mr. Heineman marshals some incredible material — including on-the-run undercover cellphone imagery — into a moving record of extraordinary individual and collective heroism, one that eventually emphasizes the personal over the political. You admire Aziz, Hamoud, Mohamad and their partisan compatriots. And soon you fear for them too, worrying over their every move as they fight an enemy that is scarily near. Yet as these men come into the focus, Raqqa and the region recede.

As the activists struggle to keep Raqqa alive for the world, funneling news to outside agencies, the dead in this movie — decapitated, crucified, shot — come close to overwhelming the living. In a few instances, Mr. Heineman shows men being shot point blank in the head; elsewhere, he cuts way before the fatal moment, which turns these murders into appalling cinematic cliffhangers. He seems to believe that it’s crucial to include numerous unfiltered ISIS atrocities, but raw and blurred versions of savagery prove this false. Mr. Heineman is clearly passionate and sincere, but he doesn’t fully grasp the power of suggestion or the audience’s capacity for empathy. You don’t need to watch a man die to understand the horror of his death.

Correction:

A picture credit on Friday with a film review of the documentary “City of Ghosts” omitted a source of the image. The photograph, of a scene from the movie, is from Amazon Studios/A&E Indie Films, IFC Films, not just IFC Films.